Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University

Glossary of Ceramic Terminology

Vince Pitelka
Copyright, 2007

ABSORPTION:  The ability of a fired clay to absorb water.  Used as a gauge of vitrification.

ACIDS:  In clay and glaze chemistry, the glass-formers which combine with bases (fluxes) and neutrals (refractories or stabilizers).

AIR-FLOATED; AIR-FLOATING: Industrial method used in processing of raw clays, where powdered clay is floated in an airstream to settle out heavier particles.

AIR-SET; AIR-SETTING:  Castable refractory or mortar which becomes hard and strong while still damp as a result chemical interaction with water. 

AIR-SHUTTER: Adjustable shutter on the rear opening of an atmosheric burner, or on the blower intake of  a power-burner, used to regulate entrainment of primary air. 

AGATEWARE:  Wares featuring swirling marbleized colors, resulting either from surface slip effects, or marbleized colored clays.

ALKALINE:  Basic, opposite of acidic - chemical nature of many fluxes. 

ALKALINE EARTHS: A category of high-fire fluxes which includes calcium, magnesium, barium, and strontium.  Encourage hard, durable, but often matt glaze surfaces.

ALKALINE FLUXES: In low-fire, boron fluxes.  In high-fire, the feldspars.

ALUMINUM SILICATE:  Broad category of materials composed primarily of alumina and silica.  Includes raw-materials such as clay and feldspar, as well as fired clay and glaze.

AMORPHOUS:  Materials such as glass, which have no regular repeating crystalline structure.

ANAGAMA: Translates "cellar kiln" - Traditional Japanese kiln evolving from the bank-kiln, and featuring a long, swelling inclined tubular ware chamber, with lower extreme serving as firebox.  Characteristically produces heavy flame-flashing and residual-ash effects. 

ANNEALING: The process of cooling a heated object gradually to allow internal shrinkage stress to equalize without damage.

APPLIQUE:  Low-relief clay shapes added to scored, slurried leather-hard surface for decoration.

ARCH BRICK:  Bricks with angled side faces, tapering across the width of the brick, which when laid together form a curved arch.  Standard arch brick give 4 1/2"-thick arch.

ASH-SLAGGING:  Heavy deposition of fly-ash onto surface of wares, furniture, and kiln interior in a wood kiln.

ASSISTED TECHNOLOGY: Studio high-production technologies adapted from industry, such as slip-casting, jiggering, and ram-pressing.

ATMOSPHERIC BURNER:  A gas burner which utilizes the effect of gas escaping from the orifice to entrain primary-air, without the need for a mechanical blower.

ATOMIC VIBRATION; ATOMIC MOTION:  Constant motion in atoms and molecules, present in all materials.  Heat accelerates atomic vibration, eventually breaking atomic bonds, causing transition from solid to liquid to gas.

AVENTURINE:  A glaze featuring iridescent metallic flecks as a result of iron crystals just beneath the surface.

BACK-BURNING:  In burners, when speed of air/fuel exiting tip of burner is slower than combustion, and flame jumps back down burner tube to fuel orifice, resulting in smoky orange flame and over-heated burner tube.

BACK PRESSURE:  In a fuel kiln, the internal pressure resulting from proper balance of combustion pressure and damper setting, resulting in more economical and efficient heatwork.

BAG WALL: Refractory wall in some fuel-burning kilns to deflect heat and flames from direct contact with the wares. 

BALL CLAY:  Secondary clays deposited in marshy areas.  Very fine particle size, high plasticity, high drying shrinkage, high in organic contaminates.  Fire white or off-white.  

BALL MILL:  A mechanically revolving vessel in which ceramic materials can be placed along with water and flint pebbles or high-fired porcelain slugs.  Used for fine-grinding of clay and glaze materials.

BANK KILN: Early form of East Asian kiln, where tubular kiln chamber was excavated in an earthen bank.  Predecessor to anagama and tube kiln.   

BANDING WHEEL:  Hand-operated turntable for applying wax resist, banded decoration, etc.

BARREL ARCH; ROMAN ARCH:  Kiln arch which forms a perfect half circle, with the arch beginning and ending on a horizontal surface, using no skew bricks. 

BASALT BODY; BASALT WARE:  Clay body with enough content of dark clays and/or metallic oxides to fire dark brown or black.

BASES:  In glaze chemistry, the fluxes or melting agents which are combined with acids (glass-formers) and neutrals (refractories).

BASO VALVE:  Safety valve used in conjunction with a thermocouple sensor probe on most gas kilns, operates on minute electrical current generated by thermocouple.  Natural-draft kilns equipped with Baso system often need no external electrical hookup at all.  In operation, button on Baso valve is depressed and pilot flame is ignited, which heats up thermocouple, energizing small electromagnet in Baso valve, which holds valve open admitting gas to main burner.  If pilot is extinguished, thermocouple cools, and Baso valve closes, shutting off gas.

BAT: Rigid flat disc of wood, plastic, or plaster placed on wheel-head.  When throwing is finished, bat is lifted off wheelhead, avoiding damage or warpage.

BEADING GLAZE; BEADED GLAZE:  A specialized controlled-crawl glaze designed to crack and crawl significantly during firing (due to extreme L.O.I. in glaze materials), and then melt into isolated beads on the surface. 

BELL KILN:  A kiln with a stationary floor but where the kiln body can be raised upwards on vertical tracks, making it easy to load and unload large forms.

BENTONITE:  Clay formed from airborne volcanic ash.  Finest particles of any clay, extremely plastic, shrinkage too high to be used alone.  2% to 3% added to clay will increase plasticity, added to glaze will help keep in suspension and improve raw glaze adhesion.

BISQUE-FIRING:  Initial kiln firing in which clay sinters without vitrifying, and though very porous, will no longer soften in water.

BLACK-BODY RADIATION:  Infrared radiation emitted from any surface or material once it reaches red heat.  

BLACK-FIGURE STYLE:  In ancient Greek ceramics, late Archaic and early Classical style in which figure first dominates, and where figures are in black against a red background.

BLACKWARE FIRING:  Bonfire-firing which is smothered with manure and/or sawdust at maximum temperature and then buried in sand, dirt, or ash, to trap smoke, causing wares to turn black. 

BLISTERING:  Glaze defect where fired glaze surface contains bubbles, which often break open to leave sharp-edged craters in surface.  Most often caused if volatization of materials in clay and glaze is still occurring when kiln is shut off, freezing bubbles in place.  Best solution in high-firing is brief oxidation soak at end of firing to allow surface defects to heal.  Same may be accomplished in low-firing by slowing down firing or by soaking kiln near maturation.   

BLOATING:  Firing defect where blisters form within claybody, raising large lumps on the surface.  Caused by expansion of gases within clay, as a result of excessive early reduction (trapped carbon - carbon coring), excessively fast bisque-firing (trapped carbon and sulfur), or over-firing (volatization of fluxes). 

BLUNGE:  The process of mixing a glaze or slip with a motorized impeller mixer.

BLUNGER:  A motorized impeller mixer used in preparing casting slip or clay slurry.  Unlike the hand-held drill-mixer, a blunger is mounted on a heavy bracket or clamp, so that slip can be left blunging for an extended period of time.

BODY REDUCTION:  Period of reduction atmosphere induced between cone 012  and 08 to bring out iron color and speckles in claybody, especially in high-fired stoneware.

BONE CHINA:  Translucent porcelain containing bone-ash.

BONE DRY:  Completely dry (and very brittle) state clay must reach before firing. 

BONFIRE FIRING - Earliest and most basic firing process, where wares are fired in an open bonfire.  See blackware firing.

BOURRY-BOX:  Type of wood-kiln fire-box where primary air enters at top of fire-box, passes down through wood, and combustion occurs at level of grates or hobs, and is supercharged by the heat of the coal-bed.  Heat and flames enter firing chamber below grate-level.  In true Bourry-box, wood can be piled above grates, and is gravity-fed down to combustion zone.

BREAKING:  In glazes, phenomenon where a glaze gives different colors in thick and thin areas - the color breaks from thick to thin.  Effect accentuated in reduction firing when glazes re-oxidize to different color in thinner areas, as in Temmoku breaking from black to brown, or copper red from red to clear. 

BURNISHING:  Method of achieving a shine by rubbing clay or slip with smooth hard object.

CALCINE:  To heat a material to a temperature high enough to drive off all chemically-combined water.

CALIPERS:  Adjustable tool for measuring inside/outside diameters, as in making lids.

CARBON-CORING:  Firing defect where excessively fast bisque-firing and/or excessive early reduction retard outgassing, causing carbon and sulfur to be trapped within claybody.  May cause discoloration of glaze, and as clay vitrifies and becomes thermoplastic, trapped carbon and sulfur may expand, causing bloating. 

CARBON-TRAPPING:  Usually purposeful effect where carbon is trapped within surface of the glaze, giving smoky shaded areas, especially in shino glazes.  Encouraged by slightly early body reduction, can be promoted in high-fire by brushing saturated soda-ash solution over glaze. 

CAR KILN; SHUTTLE KILN: Kiln where kiln floor and often the door are mounted on a car which may be rolled in and out of kiln on tracks.  Sometimes has two cars, with doors in both ends of the kiln.

CARPAL-TUNNEL SYNDROME:  Serious affliction of the wrist resulting from excessive and/or stressful repetitive-motion activity such as hand-wedging clay.  See PUGMILL.

CASTABLE:  Short for castable refractory - a refractory mix which can be cast into molds to form kiln parts.

CATENARY ARCH: A parabolic kiln arch requiring no buttressing or steel frame; laid out by hanging a chain from two points and marking the resulting curve.

CELADON:  Classic East-Asian transparent or translucent glaze with small percentages of iron and/or copper and/or chrome, giving range of soft greens, blue-greens, and gray-greens.  Most desireable Chinese celadons often contain minute air-bubble inclusions, giving slight opalescence.

CENTERING:  Critical step in throwing, occurring during and after wheel wedging, whereby the clay mass is formed into a symmetrical lump before penetrating and raising walls.

CERAMIC FIBER:  Ceramic insulating material composed of spun kaolin fibers - available in blanket form, braided tape, rigid board, and tubular flue-liners.  Highest insulating rating of standard refractories, but can release carcinogenic fibers.

CHAMOIS: Very soft, pliable animal skin - when wet works well to smooth wet clay surfaces. 

CHARGE:  A quantity of chemical material, usually salt or soda, which is inserted or injected into a hot kiln during vapor-glazing processes.

CHARGING: - Inserting or injecting a charge of chemical material into a kiln during vapor-glazing processes. 

CHEMICALLY-COMBINED WATER:  Water bonded in molecular combination within clay and glaze materials, which is driven off during the water-smoking period of the firing.

CHINA CLAY:  See KAOLIN.

CHINA PAINTS; ENAMELS:  Very-low-temperature (cone 018) glaze colors applied over a previously-fired higher-temperature glaze.  Allow greater detail, brighter colors than other ceramic glaze effects, but are vulnerable to surface abrasion.

CHINOISERIE:  European pottery, 18th century and later, featuring decoration inspired by imported Chinese Ming Dynasty wares. 

CHUCK:  On the wheel, a temporary wet-clay form or re-useable bisque-fired form upon which wares may be inverted for trimming. 

CHUN:  A pale gray-blue feldspathic stoneware glaze featuring opalescence due to inclusions of phosphorous and/or other materials.

CLAY:  Widely occurring aluminum silicate mineral resulting from natural decomposition of feldspar and granite.  Composed of microscopic disk-shaped platelates which give clay its slippery, plastic quality.

CLAYBODY:  Clay mixture formulated of clays and other ceramic raw materials to give desired working characteristics.

CLIMBING KILN:  Any one of a variety of East-Asian kiln designs featuring single or multiple chambers which climb up a slope, creating adequate draft often with little or no chimney.  See ANAGAMA, NOBORIGAMA, TUBE KILN.

CLIMBING REDUCTION; PARTIAL REDUCTION: In a fuel kiln, atmosphere which is slightly reducing, but still allowsincrease in temperature.  See REDUCTION FIRING. 

CLOSED FORM:  Wheel-thrown form which is closed off completely and then altered to form a vessel or sculptural component.

CMC GUM:  See GUM ADDITIVES.

COEFFICIENT OF EXPANSION: - A measurement of a material’s tendency to expand when heated and contract when cooled.  The higher the coefficient of expansion, the lower the thermal-shock-resistance.  In firing dissimilar materials in contact with one-another the coefficient of expansion must be matched.  See THERMAL EXPANSION. 

COGGLE: see ROULETTE.

COIL CONSTRUCTION: Ceramic forming method utilizing rope-like coils of plastic clay, assembled in successive courses to build up wall of vessel or sculpture. 

COLLARING; NECKING-IN: Process of reducing the upper diameter of a thrown form by working the walls of the rotating form inwards with fingers or rib, as in a bottle shape. 

COLLOID; COLLOIDAL: Gaseous, liquid, or solid materials which remain suspended within glaze melt without dissolving into melt, and which often coagulate to form visible particles.  An example is copper in a copper-red glaze - if firing is too fast, minute copper globules will not coagulate into visible masses, and glaze will be clear.

COLOR-ACTIVE SLIP: Slip of a composition which affects color of glazes placed over it.  Can result from coloring oxides, color modifiers, or textural qualities which influence color. 

COMBING: Decoration where a toothed instrument is dragged over a soft clay surface, sometimes through a layer of slip. 

COMBUSTION:  Reaction initiated when fuel reaches kindling temperature, at which point oxidation of hydrocarbon gases releases heat, sustaining and accelerating reaction.   

COMPRESSION:  In wheel-throwing, the act of hand or finger pressure on the clay, resulting in lower moisture content and a denser structure.  Lack of compression in bottoms of pots can result in “S”-cracks.

CONE:  See PYROMETRIC CONE.

CONTINUOUS KILN:  Industrial tunnel car-kilns or rolling-hearth kilns in which wares are slowly moved through a kiln which continuously remains at maturing temperature.

CONTROLLED-CRAWL GLAZE:  A glaze designed to crawl as glaze materials shrink during drying or during early red heat, producing alligator-skin or beaded effect.  See CRAWLING.

CONVECTION: Transference of heat through moving air currents.

CONVECTION CURRENTS: upwards rise of warm air currents due to the transference of heat. 

COOLING RAMP: The profile or schedule of temperature change in the cooling of a kiln.  See FIRING RAMP. 

COPPER RED; FLAMBE; OXBLOOD; SANG DE BOEUF:  Popular midrange and high-fire glazes featuring very small percentage of copper which, under correct light reducing conditions, gathers into colloidal particles of red copper oxide, producing bright rich red and red-purple colors.  For good copper reds, start reduction at cone 012 or 010, and maintain partial up to maturation, with oxidation cleanup at end but no glaze reduction.

COTTLES:  Adjustable wooden forms used in casting plaster molds. 

CRACKLE: see CRAZING.

CRACKLE GLAZE:  A glaze designed to craze for decorative effect. Appropriate primarily for non-functional objects and surfaces, since crazing is a flaw and weakens wares. 

CRAWLING:  Glaze fault where glaze recedes away from an area in the firing, leaving bare clay. Usually caused by dusty, dirty, or oily surface beneath glaze or by excessively powdery glaze.  In some cases results from very high L.O.I. in glaze materials, causing high glaze-shrinkage and resulting cracking during firing.  Used intentionally in controlled crawl and beading glazes.

CRAZING:  Very fine surface cracks in fired glaze surface - technically a fault in glazed wares, but often sought-after, especially in raku.

CRISTOBALITE:  Crystalline form of silica, which can form in clay and glaze above 2200 degrees F.; has very high coefficient of expansion, producing low thermal-shock-resistance.  Promoted by excessive free silica in clay and/or glaze, by repeated firing, and/or by excessive soaking or slow firing/cooling at high temperatures.

CROSS-DRAFT:  Fuel-burning kiln, usually downdraft, where heat enters at floor level at one side of ware chamber and exits at floor level at opposite side of chamber. 

CRYSTALLINE:  Solid material characterized by regular repeating geometric molecular structure or lattice, with specific melting point, as compared to glass, an amorphous, non-crystalline material which softens over broad temperature range.  See GLASS, SUPER-COOLED LIQUID. 

CRYSTALLINE GLAZES:  Glazes in which significant macrocrystalline structure forms in surface of low-alumina glaze seeded with zinc or titanium.  Crystalline glazes feature large, visible crystal development, vs. microcrystalline effects, as in matt glazes and saturated-iron glazes. 

CUERDA SECA:  Technique where a design is outlined in oxide-tinted wax resist, and the intervening spaces coated with glazes.  Finished results show areas of glaze divided by dark unglazed lines.

CYLINDER KILN:  First stage in kiln evolution beyond bonfire, consisting of cylindrical clay wall containing fire and wares, covered with layer of shards.

DAMPER:  In fuel-burning kilns, adjustable refractory plate located in exhaust flue, allowing control of back-pressure and secondary air, regulating kiln atmosphere. 

DE-AIRING:  The process of removing the air from a plastic clay mass, usually accomplished through wedging, or far more effectively with a vacuum de-airing pugmill.

DEFLOCCULATE; DEFLOCCULATION:  Process of adding an alkaline (usually) material (deflocculant) to a suspension, which introduces like electrical charges to all particles, causing them repel one another and remain in suspension.  A deflocculated suspension gives flowing consistency with less water content, meaning lower drying shrinkage - especially important in slip-casting.  Extremely low percentage of deflocculant additive is needed - 1/4 of 1% (of dry batch weight) soda ash and/or sodium silicate.

DISPERSION: Natural tendency of materials in liquid solution to go from area of high concentration to area of lower concentration, resulting in even distribution of materials throughout the glaze melt.

DISPERSOIDS: In glaze-melt, inclusions which disperse throughout the melt without actually dissolving into the glassy-phase.  Examples are titanium or zirconium particles, colloidal copper, or minute air bubbles.

DISSOLUTION: Action of a solvent material upon a solid, bringing it into liquid solution. During glaze firing, the point at which the glassy-phase dissolves the sintered structure of the glaze, producing a true glass.

DOWNDRAFT:  Kiln where exhaust gases exit through flue at floor-level.  Requires chimney to develop convection currents needed to draw off exhaust gases.  Best system for reduction firing.

DOWN-FIRING:  Practice of slowly cooling a kiln by keeping burners or elements turned on, in order to promote glaze crystal formation or other phenomena.  See SOAKING, ZONE OF CRYSTALLIZATION. 

DRAFT: The flow of exhaust gases out of a fuel kiln, affecting intake of flames and secondary air.

DRAW-RINGS; DRAW TRIALS:  In vapor-glaze firing, small rings of clay, formed to stand vertically, which are placed inside a spy-hole and may be removed with an iron rod during the firing, quenched in water, and examined to determine the degree of glaze deposition.

DRAWING:  The removing of wares from a kiln.

DRILL-MIXER:  Electric-drill-mounted impeller-mixer excellent for mixing glazes, slips, and slurries and for blunging casting-slip.

DUNTING:  Traditional term referring to serious cracking occurring in cooling, resulting from drawing too soon, from extreme excessive glaze-compression, or from low-thermal-shock-resistance in over-vitrified wares resulting from over-fluxing and/or over-firing. 

EARTHENWARE:  Low-fired ware, usually still porous after firing - must be sealed with vitreous glaze to be functional.

EARTHENWARE CLAY:  Natural low-fire secondary clay - fluxed with iron, fires porous.  Often called "common" clay, found almost everywhere, matures below 2000 degrees F. 

EFFLORESCE, EFFLORESCENCE:  Formation of crystalline deposits on surface of clay or concrete as soluble compounds migrate to surface during drying.  See EGYPTIAN PASTE.

EGYPTIAN PASTE:  A self-glazing clay body in which soluble alkaline fluxes effloresce to the surface as the piece dries, and subsequently form a thin glassy coating in the firing. 

ELEMENT:  Any of a group of slightly over 100 substances on earth which may exist as individual atoms, and from which all materials on earth are composed.  See PERIODIC TABLE OF THE ELEMENTS.  In electric kilns, the heating coils. 

ENAMELS:  see CHINA PAINTS.

ENGOBE: Slip formulated with less raw clay content in order to reduce drying shrinkage, to allow application to bone-dry or bisque-fired clay.  See UNDERGLAZE.

ENTRAINED AIR: Primary air drawn into an atmospheric burner, or mechanically injected into a power burner.   

ENVELOPE KILN:  A kiln in which the firing platform is stationary, but the body of the kiln rolls out of the way horizontally on tracks.  Often equipped with two firing platforms to be fired alternately.

ERGONOMICS: The science of comfortable and effective utility, determining how well a functional object or device works with the human body.

EUTECTIC:  Chemical phenomenon where two materials in combination melt at lower temperature than either material by itself.  For example, lithium carbonate and silica each melt at cone 32 (3100 degrees) but mixture of 55% silica and 45% lithium develops a eutectic and melts at cone 06 - 1830 degrees. 

EXTRUDER:  Machine which forces plastic clay through a die to produce extruded clay shapes.

FACETING:  Decorating technique involving cutting or paddling flat facets in the clay surface.

FAIENCE:  Widely used (and misused) term referring to any earthenware pottery glazed with an opaque glaze (usually white) and overglaze decoration.

FEATHERING; FEATHER-COMBING:  Decorating technique where a soft, fine pointed tool is drawn through adjacent contrasting-colored bands of liquid slip applied to a damp clay surface.

FELDSPAR:  A range of aluminum silicate minerals used primarily in high-fire claybodies and glazes, contributing fluxes including potassium, sodium, lithium, and/or calcium. 

FETTLING KNIFE:  Long tapered knife useful for trimming cast or pressed pieces, for separating mold components, and for many other clay studio applications.

FIGURE-GROUND: The fundamental design relationship between foreground imagery and background or surrounding area.  Foreground shapes subdivide background, creating additional important shapes.

FILLERS: See TEMPERING MATERIALS. 

FIREBOX:  The part of a fuel-burning kiln where fuel gases combust before contacting wares. Gas kilns need little if any firebox, while wood and oil kilns produce long hot flames which require a large firebox unless flame-flashing and ash-slagging (with wood) effects are sought.

FIRE CLAY:  Highly refractory secondary clays with minimal fluxes and usually fairly coarse particle size - Low shrinkage, buff-color, often non-plastic.

FIRE-EYE:  Ultraviolet sensor used on industrial burner systems to monitor burner flame.  See FLAME-RECTIFICATION.

FIRING DOWN:  Maintaining some heat input after maturation, to retard cooling, or to maintain reduction atmosphere during cooling.  See REDUCTION COOLING. 

FIRING RAMP: The profile or schedule for temperature-change in a kiln-firing, often including both the heating and cooling ramps.

FLAME-FLASHING:  Surface effects caused by direct flame contact on wares.

FLAME-OFF; BLOW-OFF:  In burners, when speed of air/fuel mixture exiting tip of burner is greater than combustion rate - flame jumps off tip of burner and often blows out.

FLAME-RECTIFICATION SYSTEM - A burner system with automated re-ignition feature which immediate restores flame should it become extinguished. Usually incorporates a fire-eye ultra-violet sensor to monitor burner flame. 

FLAME-RETENTION TIP:  Gas burner tip which causes turbulence in moving stream of gas and air, intermixing them, speeding combustion, holding flame at burner tip, preventing both flame-off and back-burning. 

FLAME-WARE: Wares made to stand stove-top heat.  Explosions from trapped moisture, and resulting lawsuits have caused studio flame-ware to disappear from the domestic market. 

FLASHING:  Color change in fired clay or slip due to direct flame contact and residual ash deposition in wood firing, or due to variable currents of vapor deposition in salt and soda firing.  Flashing can occur on almost any light-colored clay body, but is most dramatic on porcelain bodies and slips.

FLINT :  See SILICA

FLOCCULATE; FLOCCULATION:  The process of adding an acidic (usually ) substance (flocculant) which gives particles in suspension opposite electrical charges, causing them to attract one-another (to flock together) - a disadvantage in a casting slip but a great advantage in a claybody or a decorating slip.  Usually only claybodies high in kaolin need to be flocculated by adding ˝ of 1% (of dry batch weight) epsom salts.  Flocculation also often used to thicken up a glaze to help keep it in suspension and to improve application properties.

FLUE:  Passages in kiln for flames or exhaust gases.

FLUTING:  Decorating technique involving carving or forming vertical flutes or grooves in the surface of a piece.

FLUX:  Low-melting component in clay or glaze which reacts with silica to form glass.

FLY-ASH:  Airborne ash in a wood-kiln.

FOOT:  Base of a ceramic piece.

FORCED-DRAFT:  Direct-connected exhaust system equipped with suction fan, used on some commercial furnaces, but never on studio ceramic kilns.  Term often mistakenly used to refer to FORCED-AIR system.

FORCED-AIR:  Firing system in a fuel-burning kiln which uses power-driven blowers or other pressurized air-source to entrain primary air. 

FRACTURE PLANE:  Fracture zone which results when clay components are pressed straight together without disrupting the surface (by scoring and adding slurry or by smearing together) to intermix the platelates.  Parts hold together while wet and tacky, but will separate easily when dry or fired. 

FRIT:  Combinations of ceramic materials which have been melted to a glass, and crushed/ground back to a powder, in order to give greater chemical stability, and to eliminate toxicity resulting from water solubility of raw material.

FRITTING; FRITTED:  The process of melting particular ceramic materials to a glass and then crushing and grinding to form a frit. 

FUEL TRAIN: The path of fuel in the plumbing a gas or oil kiln, leading to fuel delivery within the burner.  

FUMING:  VAPOR-GLAZING process where metallic salts are introduced into kiln or onto wares at about cone 018, producing thin layer of metallic surface iridescence.

FUSION; FUSED: In the glaze-melt, the point where dissolution of sintered structure is complete, and all refractory particles are dissolved into the glaze melt, forming a fused material - one which has melted to liquid. 

GEOMETRIC STYLE:  In ancient Greek ceramics, early pre-Classical (Archaic) style featuring intricate geometric slip decoration.

GLASS:  Super-cooled liquid, which softens and hardens over broad range of temperature, and cools to form an amorphous, non-crystalline solid.  Level and rate of hardness (viscosity) controlled by temperature and by addition of fluxes and refractories, making possible the wide temperature-range of ceramic clays/glazes. 

GLASS-FORMER:  The primary material which, in combination with fluxes, forms the glass essential to all fired ceramics.   Primary glass-former at all temperatures is silica.

GLASSY-PHASE:  In heating ceramic materials, point where glass-formers and fluxes combine and soften to begin forming a glass.  See SINTERING, GLASSY-PHASE, DISSOLUTION. 

GLAZE:  Coating of powdered ceramic materials, usually prepared and applied in water suspension, which melts smooth and bonds to clay surface in glaze firing.  See INTERFACE. 

GLAZE COMPRESSION;  In high-fired wares, ideal state when clay body shrinks slightly more than glaze, putting glaze under slight compression, giving greater strength, resiliency.  See CRAZING and SHIVERING.

GLAZE-FIRING:  Kiln firing in which glazes are melted to form a smooth glassy surface.

GLAZE-FIT:  The matching of glaze to claybody in terms of composition and coefficient of expansion so that it will adhere permanently.  See GLAZE COMPRESSION, INTERFACE.

GLAZE-MELT:  The chemically-active state of the melted glaze.

GLAZE-RESIST:  Decorating technique where resist materials are applied to prevent glaze from adhering to certain areas.

GRANITE WARE: Ware with mottled slip or glaze treatment to create appearance of granite.

GRATE KILN:  An early form of kiln originating in the Middle East , with a below-ground firebox, a grate supporting the wares within a cylindrical enclosure, and a piled shard roof.

GREEN; GREENWARE:  Any dry, unfired clay form.

GREEN GLAZING; RAW GLAZING:  Glazing leather-hard or bone-dry wares for SINGLE-FIRING.

GROG: Filler or tempering grit formed by grinding high-fired clay; added to claybodies to reduce shrinkage and give structure for throwing or handbuilding.

GROUNDHOG KILN:  A long, low horizontal sprung-arch or barrel-arch wood-fired kiln, often buttressed with banked earth, traditionally used by potters on the eastern slope of the Appalachian Mountains .

GUM ADDITIVES:  CMC gum, Veegum-Cer - water soluble organic thickeners often added to glazes or slips as a suspension and/or brushing agent.  Also used to allow glaze adhesion to vitrified or previously glaze-fired surfaces.  Most commercial low-fire glazes and underglazes contain gum additives. 

HANDBUILDING:  Forming plastic clay by hand without the wheel, using pinching, coiling, and/or slab construction.

HARDBRICK:  Hard, dense firebrick generally used only in high-stress areas of kiln (floor, burner ports, flues, bagwall) and for corrosive firing processes (salt, soda, wood).

HARD-PASTE:  Traditional European term for high-fired porcelain.

HEATWORK: Work done by effective heat-transfer to wares, resulting from ramp-rate, temperature and duration of firing. 

H.E.P.A.:  High Efficiency Particulate Arrestor - industrial designation which indicates that a filter or dust-mask is capable of blocking microscopic particulates such as dangerous ultra-fine silica dust. 

HIGH-FIRE:  High-temperature firing range usually including cone 8 to cone 12, for firing stoneware or porcelain.

HISPANO-MORESQUE:  Decorative style originating in Moorish Spain, characterized by extremely elaborate patterning and curvilinear plant motifs; major influence on Italian Maiolica.

HOBS:  Steps constructed in walls of wood-kiln firebox in place of grates.  Wood cut to proper length catches on hobs and remains suspended above coal-bed, insuring quick and complete combustion.

HOT FACE: The interior refractory surface of a kiln.  

HYDROCARBONS - Combustible gases containing hydrogen and carbon, present in fuel gases, and produced when carbon-based liquid or solid fuels are heated above kindling temperature. 

HYDROMETER:  Laboratory device for measuring specific gravity in slips and glazes.  Does not give absolute measurement, and should be used only for comparing and reproducing results.  Must be an appropriate glaze hydrometer for measuring suspensions heavier than water.  Does not work well in thick glazes.

IMPRESSING:  Decorating technique where textured or patterned material or object is pressed into clay surface. 

INCISING:  Decorating technique where design is formed by cutting or carving shallow lines in clay surface. 

INCLUSIONS:  Any particles or bubbles of material which remain suspended within the glaze-melt, affecting the glaze appearance.

INSULATING FIREBRICK; IFB; SOFTBRICK:  Porous firebrick with insulating values much higher than hardbrick.

INTERMITTENT KILN; PERIODIC KILN:  A kiln which is loaded cold, brought to temperature, cooled and unloaded.  See CONTINUOUS KILN.

INTERFACE:  Contact face between clay and glaze.  On low-fired wares, primarily just a physical interlocking of glaze into pores in clay.  In high-fired wares, an intimate interaction of clay and glaze, reinforced by mullite crystals, creating very strong bond.

IRON:  Most abundant coloring material on earth, responsible for brown and sometimes black color in natural rock and fired clays.  In oxidation firings, the red ferric oxide remains very refractory, but in reduction firing and or at high-fire temperatures it transforms to the black ferrous oxide, a powerful flux. 

JASPER WARE:  Ware developed by Wedgewood in which colored stoneware is decorated with contrasting raised relief slip or clay imagery.

JIGGERING; JOLLEYING:  A partially mechanized vessel-forming process on the potter’s wheel, where a pivoting arm (a jigger or jolley) with a mounted profile rib is brought down against a centered clay ball within or upon a wheel-mounted mold.

JUG FINGER; POTTER’S FINGER: A long wooden tool with a knob on the end, often covered with chamois, which is inserted in a bottle or jug on the wheel after the top has been necked in, and is used to broaden and refine the shape. 

KAKI: High-fired Japanese saturated iron glaze usually containing at least 10% (of dry batch weight) iron oxide, which produces iridescent microcrystalline rust red.  See TESSHA.

KANNA: Japanese trimming tool, formed of a steel strap bent and sharpened on the ends.

KAOLIN; CHINA CLAY:  Primary clay which fires pure white - coarse particle size, low plasticity, high-temperature - major component of porcelain and some whiteware bodies. 

KAOLINITE: Crystalline clay mineral forming the basis of most clays we use in ceramics.  

KERAMOS:  Greek term meaning earthenware from which the word ceramics is derived.

KEYS; REGISTRATION MARKS:  In plaster slip-casting molds, small rounded depressions carved in each parting face of a mold segment, so that when adjacent segments are cast a corresponding bump will form.  In use, the keys allow the mold segments to be aligned and assembled securely after the original prototype is removed.

KILN FURNITURE:  Refractory shelves, posts, and stilts used in a kiln to support the wares.

KILN SITTER:  Automatic-shut-off device mounted on many electric kilns; accepts a small-size pyrometric cone, and shuts off kiln when cone deforms.

KILN WASH :  Refractory slip coating applied to top surface of kiln shelf to protect from glaze runs.  For all but salt and wood firings, 50-50 kaolin and silica.  For salt and wood, 60% alumina, 30% kaolin, 10% ball clay.

KINDLING TEMPERATURE:  In combustion of fuels, temperature which must be reached before oxidation reaction will produce enough heat to sustain and accelerate.  For wood and paper, kindling temperature is 451 degrees F.

LATEX: A liquid rubber product often used as a resist.  It has the advantage of being much more durable and insoluble than wax resist, and can be peeled off after functioning as a resist.  See RESIST.    

LEATHER-HARD:  Condition of clay where it has stiffened but is still damp.  Point at which pieces are joined and most surface modification and trimming are done.  Soft-leather-hard ideal for forming, joining, thick slip-decoratin.  Medium-leather-hard good for thin slip-decorating, joining, incising, carving, piercing.  Hard-leather-hard good for thin slip-decorating, carving, scraping.

LIMIT FORMULAS:  A set of formulas giving ideal ranges of glaze oxides which can be present in glaze unity formulas at different firing temperatures.  Used in conjunction with glaze calculation to predict or evaluate glaze performance.

LINE BLEND:  Method for testing glaze materials, where proportional amounts vary through a series of samples between two limits.  May involve change in a glaze material, or addition of a colorant or modifier.  See TRIAXIAL BLEND.

LOSS ON IGNITION; L.O.I.:  The fraction of any material which volatizes and outgases during firing - includes carbonates, sulfates, nitrates, organic contamination, and chemically combined water. 

LOW FIRE:  Low-temperature firing range, usually below cone 02 (2048 degrees F.), used for most bisque-firing, and for glaze firing terracotta and whiteware. 

LOW-MID-RANGE:  Firing range usually including cone 01 to cone 3, under-used in studio ceramics, useful for functional earthenware, refractory sculpture bodies, and outdoor terracotta work.

LUSTER:  Metallic overglaze finish created either by painting prepared luster (metallic salt in organic binder) over previously-fired glaze and firing to cone 018, or by spraying metallic salt dissolved in water into kiln and/or on to wares at low red heat, either during cooling cycle of a glaze-firing, or in a separate firing heated to that temperature. 

LUTING: English word for attaching clay forms with slurry.

LYE:  Potassium Hydroxide or Sodium Hydroxide.  Caustic alkaline soluble which leaches out of wood ashes when they are soaked in water.  Occasionally used as a flux additive in terra sigillata, as in the black slip on Classical Greek wares.  Toxic in liquid state, and may cause burns.

M.S.D.S.; MATERIAL SAFETY DATA SHEET, available from your supplier, giving toxicity information on each of the ceramic materials you use.

MACROCRYSTALLINE:  Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects.  See CRYSTALLINE GLAZE. 

MAIOLICA: Earthenware, generally terracotta, with opaque white glaze and colored overglaze decoration, not to be confused with majolica.

MAJOLICA:  Historic English style of brightly glazed earthenware vessels made in the form of vegetables, animals, etc., not to be confused with maiolica. 

MANOMETER:  Gauge for measuring water column inches.  Primitive glass "U-tube manometer" consisting of a “U”-shaped glass tube partially filled with water, with the pressure source connected to one end.  See WATER COLUMN INCHES.

MATT GLAZE:  Glaze featuring a dull, non-glossy surface.

MATURITY; MATURING POINT:  Firing point at which a claybody or glaze reaches it's desired condition of color, hardness, density, etc.

MICROCRYSTALLINE: Crystal effects dependent on tiny crystals in the glaze surface, as compared to MACROCRYSTALLINE effects. 

MID-RANGE:  Glaze-firing range usually including cone 4 to cone 7, very popular with electric kilns. 

MISHIMA:  East Asian method of creating an inlaid effect by applying contrasting slip into a design incised in leather-hard clay.  When the slip stiffens, the excess is scraped off. 

MODIFIERS:  Glaze materials which are added to a base glaze to modify surface qualities such as color, gloss, mattness, opacity, crystal development, etc.

MOLD-RELEASE COMPOUNDS - In making plaster molds, compounds which are applied to all surfaces except damp clay, in order to prevent plaster from sticking.  Liquid hand soap works great.  Paint it on and let it dry before pouring plaster.  Never use oil-base release compounds on plaster molds for press-molding or slip-casting. 

MOLECULE: A group of atoms chemically bonded together to form a compound. 

MONTMORILLONITE:  Clays such as bentonite, resulting from the breakdown of airborne volcanic ash.  The finest particle-size of all clays.  Very high shrinkage, and generally used only as an additive to claybodies or glazes.  See BENTONITE.

MOUSE-HOLE:  In some wood kilns, a small air-port and passage which admits air directly beneath the coal-bed.

MUFFLE KILN:  A fuel-burning kiln in which flames and combustion gases are ducted through sealed refractory flues within the ware chamber, heating the wares while maintaining a clean oxidizing atmosphere.

MULLITE:  Very desirable aluminum silicate crystals which form above 1800 degrees F. – greatly adding to the strength of high-fired vitrified claybodies and glazes.  In well-formulated high-fired wares, clay-glaze interface is extremely strong partially due to interlocking mullite crystals.

MULTI-FIRED:  Wares where more than one glaze firing is required to achieve particular surface effects, as in lusters and china paints.

NATURAL DRAFT:  Firing system in a fuel-burning kiln which uses natural convection currents in kiln (updraft) or in chimney (downdraft), without any mechanical blowers, to draw in secondary air, circulate heat and atmosphere through kiln, and exhaust combustion gases.

NECKING-IN: See COLLARING. 

NEUTRAL ATMOSPHERE:  In fuel-burning kiln, atmosphere which is neither oxidizing nor reducing - most efficient atmosphere for efficient climb and even temperature.

NEUTRALS:  In glaze chemistry, the refractories or stabilizers which are combined with bases (fluxes) and acids (glass-formers). 

NOBORIGAMA:  Translates "step-climbing kiln" - Traditional Japanese multi-chamber downdraft/crossdraft climbing kiln, with initial firebox at bottom, and secondary fireboxes in each chamber.  As kiln is fired, each chamber preheats the next.  See SEQUENTIAL FIRING. 

NUKA: An often irridescent milky white Japanese glaze containing rice-hull ash.

OPACIFIER:  In glaze formulation, a material which produces inert inclusions or minute crystals in glaze, causing it to become opaque.  Most common are tin and zirconium silicate.

ORIFICE:  In a gas or oil burner, the restricted opening through which a jet of fuel emerges.

OUT-GASSING: The escape of gases from clay and glazes during firing.  Carbonates, sulfates, nitrates, organic contaminants, and chemically-combined water volatize between 600 and 1200 degrees F. Other compounds, especially fluxes, volatize later during glaze-melt.  See WATER-SMOKING PERIOD, CARBON CORING, OXIDATION CLEANUP. 

OVERGLAZE:  Any surface decoration applied over the glaze surface, either as an oxide wash applied over raw glaze surface before glaze-firing, or as a lower-temperature medium fired onto a previously higher-fired glaze surface, as in china paints and lusters..

OVERSPRAY:  In spraying glazes or other mediums, the small droplets and/or dust which do not settle on the object being sprayed.  An adequate spray-booth is essential in order to exhaust all overspray.

OXIDE:  A molecule combining any element with oxygen. 

OXIDE STAIN: A mixture of coloring oxide and water, sometimes including a little flux, used as an overall patina (often on unglazed work), or for overglaze brushwork.  See PATINA.

OXIDIZE:  To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions. 

OXIDATION:  Any chemical reaction in which atoms or molecules combine with oxygen atoms.  The combustion of fuel is an oxidation reaction. 

OXIDATION CLEANUP; OXIDATION SOAK:  When reduction firing reaches maturing temperature, a period of oxidation to allow outgassing to stop and glazes to heal. 

OXIDATION FIRING: Any kiln atmosphere with an abundance of oxygen to combust the fuel and oxidize the ceramic materials.  Includes all electric firings, and any gas firing with adequate air to insure complete combustion of the fuel close to the burner.  

PADDLE AND ANVIL:  Forming technique where a soft-leather-hard form may be shaped and thinned with a wood or bisqued clay paddle on the outside, and a rounded “anvil” of wood or bisqued clay applying corresponding pressure on the inside.

PADDLING:  Technique of shaping a soft or medium-leather-hard piece by gently hitting with a wooden paddle (sometimes textured) to create flat facets or to resolve irregularities in the surface.

PAPER CLAY:  Technique popularized by Rosette Gault, utilizing a clay body or slip containing paper pulp, which reduces shrinkage in drying stage, and encourages extremely strong joinery, allowing unconventional joinery such as wet to dry. 

PAPER RESIST: Decoration technique where strips of moist or adhesive paper is adhered to the surface to resist application of slip or glaze. 

PARTIAL REDUCTION: see CLIMBING REDUCTION. 

PATE-SUR-PATE: Paste-on-paste - decoration in which sculptural relief decoration is built up with contrasting slip on surface of leather-hard ware.

PATINA:  An overall thin wash of glaze or oxide stain, allowing the color and texture of the claybody to show through. 

PEEP-HOLE; PEEP:  See SPY-HOLE.

PERIODIC KILN:  See INTERMITTENT KILN.

PERIODIC TABLE OF THE ELEMENTS:  A table listing all of the chemical elements (those substances which can theoretically be reduced to individual atoms), with the smallest and simplest across the top, and the heaviest and most complex at the bottom, and grouped by similar physical/chemical characteristics. 

PINCHING:  Hand-building method where clay objects are formed by pinching repeatedly between thumb and fingers, or between fingers of one hand and palm of opposing hand.

PINHOLING:  Glaze defect characterized by fine pinholes in the surface - often caused by pinholes already present in dry unfired glaze coating.  Can also be caused by burst bubbles in glaze surface which are not given opportunity to “heal” at end of firing.

PIT FIRING:  A type of bonfire-firing where wares are buried in sawdust in a pit in the ground, and a bonfire is built on top, so that the fire and coals slowly burn away the sawdust and fire the wares.  Not to be confused with SAWDUST SMOKING. 

PLASTICITY: Quality of moldable flexibility in damp clay - superior plasticity depends on smaller clay particle size, slight acidity, less non-plastic additives, aging of damp claybody, adequate water content, and/or addition of accessory plasticizers, such as veegum-T or macaloid.

PLASTICIZERS:  Materials added to some claybodies, especially those high in kaolins, to increase plasticity and dry strength - includes bentonite, macaloid, Veegum-T. 

PLATELATES:  Flat, thin crystals which make up clay.  When wet they become sticky and slippery, creating the phenomenon we call plasticity.

POLISHING:  As compared to burnishing, the act of creating a shiny surface on terra sigillata (or any clay or slip) by rubbing with soft cloth, a soft brush, or a piece of plastic film.

PORCELAIN:  High fired vitreous claybody containing kaolin, silica, fluxes, and often ball clay to increase plasticity, with total clay component not more than 50%.  Usually pure white or “eggshell” in color, some porcelains may fire translucent where thin.

PORCELAINEOUS: White-firing stoneware claybodies closely related to porcelain.

POSTS: Refractory columns used as kiln furniture to support kiln shelves.  See STILTS.

POST-FIRING SMOKING:  See RAKU.

POTTER’S FINGER: see JUG FINGER.

POWER BURNER:  Any burner in which a mechanical blower or other pressurized air source entrains the primary air for combustion.

PRESSING:  Forming of multiples by pressing plastic clay into a plaster press-mold.

PRIMARY AIR:  In a fuel-burning kiln, air which feeds initial combustion - in a wood-burning kiln, air which feeds base of flames.  In a gas kiln air which enters back end of burner tube via atmospheric pressure in an atmospheric burner or via blower pressure in a power burner.  See SECONDARY AIR.

PRIMARY CLAYS; RESIDUAL CLAYS: clays which remains at the site of the parent rock, without being transported by wind or water - tend to be free of contaminants but coarse particle size and therefore low plasticity.  Includes the purest kaolins.

PUG-MILL:  A machine similar to an over-sized meat-grinder, used to homogenize plastic claybodies.  De-airing pugmill has vacuum pump attachment, effectively removes all air from clay, eliminating need for hand-wedging.  See CARPAL TUNNEL SYNDROME. 

PUMICE: See VOLCANIC ASH

PYROMETER:  Temperature gauge connected to a thermocouple, which indicates temperature within kiln.  Pyrometers provide good general reference, but only respond to temperature, while clay and glazes (and cones) are affected by temperature, duration, and atmosphere of firing.  See PYROMETRIC CONES. 

PYROMETRIC CONES:  Small slender pyramidal-shaped indicators made of ceramic material formulated to bend at a specific temperature - standard method for determining maturing temperature of firing.  Like clay and glazes, cones respond to temperature, duration, and atmosphere of firing, far more accurately than mechanical measurement.

PYROPLASTIC:  Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase. 

QUARTZ: see SILICA.

QUARTZ INVERSION:  Abrupt expansion in heating and corresponding contraction in cooling which occurs in silica crystals in all clay and glazes at around 1063 degrees F.  Greatest risk is in cooling high-fired wares, and in re-firing previously high-fired wares. 

RAKU:  In the West, firing process inspired by traditional Japanese raku firing.  In American raku, work is removed from kiln at bright red heat and subjected to post-firing smoking by placing in containers of combustible materials, which blackens raw clay and craze-cracks in glaze.

RAMP:  Profile of the firing of a kiln, including speed, duration, soaking periods, etc. of both the heating and cooling cycle, as in firing ramp and cooling ramp.

RAW GLAZING:  See GREEN GLAZING. 

RE-CRYSTALLIZATION: During initial cooling, the formation of microcrystalline or macrocrystalline structure in the glaze surface

RED-FIGURE STYLE:  In ancient Greek ceramics, Classical stage of vase-painting, featuring figures in red against black background, with highlights painted in black and white.

REDUCE:  To induce reduction.

REDUCING AGENT:  Material such as silicon carbide used in clay or glaze to create localized reducing conditions in an oxidation firing.

REDUCTION:  Chemical reaction in which oxygen atoms are removed from a compound.

REDUCTION COOLING:  In a fuel kiln, maintaining slight reduction atmosphere during cooling cycle, from maturing temperature down to approximately 1400 degrees F., in order to minimize re-oxidation of clay and/or glazes.  See RE-OXIDATION. 

REDUCTION FIRING: In fuel-burning kilns, firing atmosphere with insufficient oxygen to completely combust fuel, introducing abundance of un-oxidized carbon and hydrogen, which extract oxygen molecules from surface of wares, altering appearance of clay and glaze.  See CLIMBING REDUCTION. 

REFRACTORY:  Capable of withstanding very high temperatures.

REFRACTORIES:  Any materials highly resistant to the effects of heat.  In kiln construction, high-temperature insulating materials, such as firebrick, ceramic fiber, etc.  In clay and glaze chemistry, the neutrals or stabilizers which are resistant to melting, and which combine with the fluxes (bases) and glass-formers (acids).  Primary refractory material in ceramics is alumina, most often introduced in the form of clay (aluminum silicate).

REGISTRATION MARKS: See KEYS. 

RESIDUAL ASH: In a wood kiln, the surface effects created by fly-ash settling upon the wares.  Effect is maximized by turbulence in firebox, considerable draft through kiln, and/or active stirring of the coal-bed.  In short wood-firings ash doesn't have time to melt, and leaves a rough, crusty surface.

RESIDUAL SALT/SODA:  In a well-used salt/soda kiln, the light salt/soda effect which may be obtained by firing wares in the kiln without introducing any additional salt or soda, due to the re-volatization of salt/soda from the walls of the kiln.

RESIST:  Material used in glazing and decorating which can be applied to surface to prevent adhesion of slip or glaze.  Resists may be in the form of tape or adhesive-backed paper, or liquid.  Common liquid resists include water-emulsion WAX RESIST, hot wax, and LATEX.  See WAX RESIST, LATEX, SHELLAC RESIST.  

RHEOSTAT:  Variable electric switch often found on forced-air burners to control blower speed.  Small inexpensive rheostats used on most power-burners are imprecise, and adjustments should always be made on basis of flame at burner tip, preferably with an air-shutter on the blower-intake for fine adjustments.

RIB:  Wide, flat hand-held tool used to shape, smooth, and/or scrape clay surfaces; usually wood, rubber, plastic, or metal, either rigid or flexible, with straight, curved, or profiled edge. 

ROMAN ARCH:  See BARREL ARCH.

ROLLED EDGE:  Beveled edge obtained by rolling the outer edge of the foot of a soft-leather-hard pot at an angle against a hard flat surface, or by paddling the edge.

ROULETTE; COGGLE: Small stamp-wheel with raised pattern around the rim, which when rolled along a plastic clay surface leaves a band of relief pattern.  Usually formed with damp or dry clay and bisque-fired.

“S”-CRACKS: “S”-shaped cracks which occasionally appear in the bottoms of wheel-thrown pots, resulting from inadequate compression of the bottom, and/or excessive water left in bottom.  Occur most often in fine-grain gritless claybodies, especially thrown off the hump. 

SAGGER; SAGGAR:  A refractory clay enclosure in which wares are fired.  Originally developed to protect wares from ash-slagging and flame-flashing in wood firings.  In contemporary use, with clean-burning gas firings, process used in exactly opposite way.  See SAGGER FIRING. 

SAGGER-FIRING:  Contemporary firing process in which wares are placed within refractory saggers, along with chemicals and combustibles, in order to achieve certain surface effects.

SALT FIRING; SALT GLAZE:  Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature.  Salt vaporizes, and sodium vapor combines with silica in clay surface, forming extremely hard sodium-silicate glaze.  See SODA FIRING. 

SAND:  Granular silica.  Major grit besides grog used to give claybodies structure for throwing and handbuilding.

SATURATED IRON GLAZE: see KAKI.

SAWDUST SMOKING; SAWDUST FIRING:  Cosmetic smoking process where unglazed wares are buried in sawdust in a brick enclosure or a perforated steel drum.  Sawdust is ignited at the top, and allowed to smoulder down over a period of hours.  Often mistakenly referred to as pit-firing, but does not achieve true firing temperatures, and wares must be bisque-fired first. 

SCORING:  Process of incising shallow grooves into surface of wet or leather hard clay in cross-hatch pattern before applying slurry and joining pieces.

SECONDARY AIR:  In fuel burning kilns, air which enters kiln after initial combustion and feeds flames as they leave primary combustion source.  In gas kiln, secondary air enters burner port around burner-tip.  In wood kiln, secondary air enters above grates in conventional fire-box, and below grates or hobs in Bourry-box.  See PRIMARY AIR. 

SECONDARY CLAYS; DEPOSITED CLAYS; SEDIMENTARY CLAYS:  Clays which have been transported away from their point of geologic origins by wind or water.  Finer particle-size gives greater plasticity - ball clays, stoneware clays, fireclays, etc.

SECONDARY KAOLINS: Kaolins which have been transported some distance from the parent rock and are still very pure, but much more plastic than primary kaolins. 

SEQUENTIAL FIRING: Firing in a sloped tube kiln or noborigama, where as each chamber or zone reaches maturity, the fire is moved up to the next, and each one preheats subsequent chambers or zones. 

SET:  To place wares in a kiln.  Or, in a loaded kiln, the entire structure of shelves, furniture, and wares.

SETTING:  Process of loading a kiln, or technically, of placing the set.  See SET.

SGRAFFITO:  Decorating technique achieved by scratching or carving through a layer of slip or glaze (helps to apply wax-resist over glaze before carving) before firing to expose contrasting claybody beneath.

SHARD:  A broken fragment of pottery.

SHELLAC RESIST:  Resist technique using shellac, which gives an extremely durable resist surface.  Common technique involves application of design with shellac over leather-hard clay, followed by aggressive damp sponging, removing clay, leaving resisted areas in raised relief.  

SHINO:  Classic Japanese glaze ranging from gray to white to orange, often containing spodumene or other source of lithium, and/or nepheline syenite.  The orange color is achieved with thinner glaze coating when fluxes in the glaze activate iron content in the claybody. In the West, many potters seek shinos which break from off-white to orange, often with carbon-trapping effects.  Carbon trapping can be accentuated with a brush coat of saturated soda-ash solution.   

SHIVERING:  Serious and dangerous glaze defect where excessive glaze-compression causes small razor-sharp chips of glaze to pop off along outer edges, corners, and rims.  All wares showing shivering must be destroyed.  Cure is to slightly increase flux and/or decrease silica in glaze. 

SHORT:  Clay with insufficient plasticity - tends to fragment during forming. 

SHRINKAGE:  Permanent contraction of the clay in both drying and  firing stages.  Overall may be as much as 18%.

SHUTTLE KILN:  See CAR KILN.

SIEVE:  A container with fine-mesh wire screen in the bottom, available in different mesh-sizes, used for straining slips and glazes.

SIEVING: The process of working a glaze or slip through a wire-mesh sieve to strain out impurities, and to break up clumped raw materials. 

SILICA; SILICON DIOXIDE; FLINT ; QUARTZ:  The primary glass-former in clay and glazes.

SILICON CARBIDE:  Extremely hard, refractory material used to form kiln shelves.  Highly resistant to corrosive atmospheres, and therefore suitable for salt, soda, and wood firing.  Silicon carbide kiln shelves conduct electricity, and should never be used in electric kilns.  Occasionally used in powdered form as local reducing agent in oxidation firings.

SINGLE-FIRING; GREEN FIRING:  Process of glaze-firing glazed greenware without a bisque-firing. 

SINTERING:  In heating clays and glazes, a solid-state reaction where particles stick together permanently, and mass can be considered fired.  With increasing temperature, fluxes and glass-formers begin to interact, increasing the strength of bond between refractory particles, still leaving open, porous structure, as in bisque-fired and low-fired wares, which are sintered but not vitrified.  

SKEW BRICKS:  Special angled bricks used to support the first course of arch bricks on either side of  a sprung arch.

SLAB-ROLLER:  A mechanized but usually manually operated device for rolling out large uniform slabs of clay.

SLAKING; SLAKE DOWN:  The process of returning dry unfired clay to a slip by soaking in water.

SLIP:  Clay suspended in water, usually the consistency of thick cream.  May be colored and used to decorate surfaces, or may be cast into plaster molds to create ceramic forms.

SLIP-CASTING:  The creation of ceramic forms by casting slip in plaster molds.  See ASSISTED TECHNOLOGY.

SLIP CLAY:  A naturally-occurring clay which contains a high enough component of flux to form a glaze at high-fire temperatures.

SLIP GLAZE:  A glaze with slip-clay or earthenware clay contributing the primary flux.

SLIP-RESIST:  Decorating technique where resist materials are applied to prevent slip from adhering to some areas.

SLIP-TRAILING:  Application of decoration to wet or leather-hard clay by flowing on lines of slip with a fine pointed dispenser, such as a rubber syringe.

SLUMP-MOLD: A mold over which a moist clay slab is slumped in order to create a vessel form.

SLUMPING - Process of creating ceramic wares with a slump-mold.  Firing defect where glassy-phase begins to dissolve sintered structure in clay, causing it to sag and deform.

SLURRY:  Very thick clay slip, often used for joining clay pieces after scoring surfaces.

SOAP:  A specialized hardbrick shape, half the width of a standard brick.  Useful for posts in salt and wood firings.

SOAKING:  During firing or cooling-ramp, the act of holding kiln at steady temperature for a period of time to allow proper formation or maturation of certain clay and glaze effects. See ZONE OF CRYSTALLIZATION, DOWN-FIRING.

SODA FIRING; SODA GLAZE:  Vapor-glazing process touted as modern-day non-toxic replacement for salt-firing.  Gives slightly less gloss and orange-peel.  Instead of salt, soda ash (sodium carbonate) in water solution is sprayed into kiln at maturing temperature, and sodium vapor combines with silica in clay to form sodium-silicate glaze.  Recent research indicates that effluent gases are highly corrosive, and no more benign (or destructive) than salt-firing fumes.

SOFTBRICK:  See INSULATING FIREBRICK.

SOFT PASTE:  Traditional European term for a dense, white claybody which fires at lower temperature and was the precursor to true porcelain.

SOLID-STATE REACTION:  Chemical reaction occurring while materials are still in their solid state.  Initial SINTERING is a solid state reaction.  

SOLUBLE SALTS:  Range of soluble metallic salts like bismuth subnitrate, silver nitrate, copper sulfate, cobalt sulfate, ferric chloride, and stannous chloride, used to create fumed luster effects on glazed wares, and to create surface effects on unglazed sagger-fired wares.  Most are soluble and highly toxic.

SOLUTION:  A liquid mix where the components are fully dissolved (as in a water/soda-ash solution for vapor-glazing), where materials will not settle out via gravity. 

SOLVENT: An agent which acts to accelerate the dissolution of a more resistant material.  In glaze-maturation, the flux-glass-former combination acts as a solvent upon alumina.

SPALLING:  Defect in repeatedly fired clay materials where outer layers shrink and peel away - most often occurs in kiln hot-face when refractories are repeatedly fired beyond their rated temperature.

SPECIFIC GRAVITY:  The weight or density of a liquid measured in proportion to that of water.  A glaze with a specific gravity of 1.2 is 1.2 times as dense as water per unit of measure.  See HYDROMETER.

SPLIT :  A specialized hardbrick, half the thickness of a standard brick.

SPOOZE:  A repair medium for bone-dry greenware, made of equal parts vinegar and corn-syrup mixed with dry powdered claybody to the desired consistency.  Name was coined by the late Canadian ceramicist Peggy Heeer.  

SPRAYING:  Application of liquid slip, engobe, glaze, or stain, using mechanized spray equipment.  Gives smooth glaze coating, but with less glaze-pooling in recesses.

SPRAY-BOOTH:  Open-front enclosure with an exhaust fan at the rear, designed to draw off all overspray and other toxic dust or fumes.

SPRIGGING:  Surface decorating technique in which small coils or balls of clay are affixed to the damp or leather-hard surface, usually with a layer of slip.

SPRUNG ARCH:  Kiln arch representing less than 180 degrees of curvature, and requiring buttressing to support outward thrust of the arch. 

SPY-HOLE; PEEP-HOLE:  Small holes in kiln door or wall, allowing viewing of cones, flames, wares, and/or atmosphere during firing.

STABILIZERS: See refractories.

STAIN:  Commercial ceramic colorants which have been fritted in order to eliminate solubility problems and give greater stability in firing and truer color before firing.  Mixture of ceramic stains or pure coloring oxides (sometimes with a little flux) in water suspension, which can for overglaze brushwork, or as a patina on unglazed clay. 

STILTS:  Term often applied to all kiln posts, but more correctly referring to specialized refractory furniture pieces equipped with ceramic or metallic points designed to support fully-glazed wares during firing.

STONEWARE:  High-fired vitreous ware, literally as hard and durable as stone.  Matures from 2200-2400 degrees F. (cone 5-11).

STONEWARE CLAY:  Naturally occurring refractory clays with adequate fluxes to fire in stoneware temperature range.

SUPER-COOLED LIQUID:  A material which, in its solid form, maintains the amorphous physical structure of a liquid rather than the ordered structure of a crystalline material.  A crystalline material softens or solidifies at a specific freezing point, while a super-cooled liquid like glass remains amorphous, softening or solidifying gradually over a broad temperature range. 

SUSPENSION:  A liquid mix where insoluble particles are distributed throughout without dissolving, and may settle out from gravity, as in a glaze or slip.

TEMMOKU:  Classic East-Asian high-iron gloss glaze giving black where thick, breaking to brown or red-brown where thin. 

TEMPERING MATERIALS; TEMPER; FILLERS:  Gritty materials like volcanic ash (low-fire only) sand, or grog, added to clay to open up body, give physical structure in wet-working, and increase thermal-shock resistance. 

TERRACOTTA:  Low temperature, porous earthenware claybody, fires red-brown due to high iron content which also fluxes clay, making it the most durable low-fired clay after firing. 

TERRA SIGILLATA:  Ultra-refined clay slip which can give a soft sheen when applied to bone-dry wares, and if polished or burnished while still damp may give a high gloss.  All ancient Greek red-black pottery and Roman red wares were finished with this technique, without the use of glaze.  See my article on terra sigillata.  

TESSHA:  A Japanese glaze similar to kaki, in which re-oxidation and crystallization of iron in the glaze results in significant areas of purplish-red, often with some black or brown surface showing.

THERMAL EXPANSION:  The physical expansion and contraction which accompanies the heating and cooling of most materials.  See COEFFICIENT OF EXPANSION. 

THERMAL SHOCK:  Effect of sudden temperature-changes during firing or during subsequent heating and cooling in daily use.

THERMAL-SHOCK RESISTANCE: Resistance to the damaging effects of thermal shock.

THERMOCOUPLE:  Temperature probe which produces minute variable electrical current dependent on degree of heat exposure - used in conjunction with pyrometers and Baso valves.

THERMOPLASTIC:  see PYROPLASTIC.

THROWING STICK:  Wooden tool used by Japanese potters as an extension of the hand for raising the inside of pots, especially tall narrow forms.

TOMBO: A “t”-shaped Japanese throwing gauge, used to measure the depth and rim-diameter of a vessel, usually when throwing off the hump.

TRIAXIAL BLEND:  Method for testing three-way combinations of glaze materials, where proportional amounts vary through a series of samples between three limits.  May involve change in glaze materials, or addition of colorants or modifiers.  See LINE BLEND.

TRIMMING:  At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools. 

TUBE BURNER:  A simple atmospheric burner with no venturi effect present.

TUBE KILN: Sequential-firing Korean kiln design evolving from Chinese bank kiln, with long sloping tubular firing chamber.. 

UNDERCUT:  Common flaw in plaster or bisque molds, where the clay or casting catches and will not pull free without breaking or distorting.  See DRAFT.

UNDERGLAZE; ENGOBE:  Colored slips formulated to have low drying shrinkage, allowing application to bone-dry or bisque-fired surface before glazing.  Commercial underglazes are available in a wide palette of colors primarily for low-fire, but many will survive high-fire.

UNDERGLAZE DECORATION:  Process of applying any decoration to the bare, (usually bisque-fired) clay surface directly before glazing.

UNDERGLAZE PENCILS:  Underglaze pigments in pencil form, excellent for marking wares and test-tiles, and for "pencil-drawn" decorative effects. 

UNITY FORMULA; SEGER FORMULA:  A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers.

UPDRAFT KILN: Kiln in which exhaust gases exit through flue in the roof of the kiln.

VAPOR GLAZING:  Any glaze process where the glaze results from vapor deposited within the kiln - includes salt-glazing, soda-glazing, and fuming.

VENTURI BURNER:  Natural-draft gas burner featuring streamlined restriction in burner tube, which increases entrainment of primary air, and the efficiency of gas-air mixing. 

VISCOSITY; VISCOUS: Reference to a material’s resistance to flowing.  A viscous glaze flows less.  As glass is heated it slowly becomes less viscous over a broad temperature-range.

VITREOUS; VITRIFIED; VITRIFICATION:  Fired clay which has fused together completely, so that the pores between refractory particles are filled with glass, and the body is impervious to water.  Vitrification is sintering in the presence of a fully-developed glassy-phase. 

VITREOUS ENGOBE: An engobe containing sufficient flux to form to a vitreous clay coating. 

VOLCANIC ASH, PUMICE:  Fine volcanic particulate expelled from surface vents in a volcanic eruption.  Similar to feldspar in composition, but usually contains iron. 

VOLATIZATION; VOLATIZE:  Change from solid or liquid to gas during during firing, resulting in outgassing from the clay or glaze.  See OUTGASSING, BLISTERING, WATER SMOKING PERIOD.  

WADS; WADDING:  Small balls or rolls of refractory clay mixture (40 alumina, 10 ball clay, 50 kaolin) placed under wares and posts, and between pots, lids, etc., in vapor-glazing and wood firing processes, to keep surfaces from sticking together. .

WARPING:  Distortion of clay forms caused by uneven stresses within clay due to forming method, uneven drying, uneven support in firing, or uneven or excessive heat in firing.

WASTER:  European term referring to a fired piece discarded due to damage in firing.

WATER COLUMN INCHES; W.C.I.: Standard low-pressure measure of gas pressure, based on pressure required to lift a column of water.  Household natural gas pressure is usually 7" or 7 W.C.I.  Industrial natural gas pressure may be 12" to 15" or more.  Propane household systems usually carry 11" of pressure, but propane kiln systems may be equipped with adjustable regulators delivering up to 20 P.S.I. (pounds per square inch). See MANOMETER.  

WATER OF PLASTICITY: Amount of water required to bring a dry clay to its state of ideal plasticity.  Common clay test used to determine this amount.  The more water needed, the finer the particle size, the more plastic the clay, and the greater the drying shrinkage.

WATER SMOKING STAGE:  The stage during bisque-firing or single-firing, below 1200 degrees F., when carbonates, nitrates, sulfates, organic contaminants, and chemically-combined water volatize and outgas.  See CARBON-CORING, OUTGASSING

WAX RESIST:  Melted wax or wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.  See RESIST.  

WEDGE BRICK:  Bricks with angled side faces, tapering along the length of the brick, which when laid together form a curved arch.  Standard wedge brick give 9"-thick arch.

WEDGING:  Process of kneading the clay with the hands to remove air-bubbles and insure homogenous mass.

WHEEL-WEDGING:  Working the clay up and down in a cone shape on the wheel to align the platelates in a spiral formation and thus increase control in centering and throwing.

WHITEWARE: Low-temperature white claybody. 

ZIRCONIUM OXIDE: Common opacifier, often used in place of the more expensive tin oxide.  Sold as Zircopax, Opax, Superpax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white.  Zirconium particles are extremely refractory, and remain as inert inclusions in the glaze-melt.  They will not affect mattness or gloss, but will increase viscosity. 

ZONE OF CRYSTALLIZATION: Period during cooling-ramp when a particular material tends to crystallize out of glaze-melt.  See SOAKING, DOWN-FIRING.  

 

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